Survey of the yeast population inside wine barrels and the effects of certain techniques in preventing microbiological spoilage

Barrels play an important role in winemaking, giving wines a specific flavour and assisting the ageing process. However, the porosity and inertness of wood encourage the proliferation of spoilage yeasts which can severely affect wine quality. The yeast microbiota resident inside barriques located in a traditional Tuscan winery were identified by partial sequencing of the 26S rDNA gene. Most of them belonged to the Saccharomyces , Candida and Pichia genera, while Brettanomyces/Dekkera represented a limited part of the population. The ability of the isolated yeasts to spoil wine through the production of volatile phenols was evaluated. Brettanomyces showed significant production of ethylphenols, with a conversion rate of over 70%; many other species of yeast showed extensive production of vinylphenols, which can have a negative impact on wine quality. To improve the microbiological control of barrels, 4 treatments were tested for their efficacy in eradicating the yeast inside the barrels: aqueous steam (100 °C, 5–30-min treatment), UV irradiation (36-W lamp, 5–30-min treatment), gaseous O 3 (40 mg/m 3 , 30-min treatment) and aqueous O 3 (2 mg/L, 30-min treatment). Steam and O 3 were the most effective treatments, eliminating about 70% of yeasts. UV appeared to be less effective, probably due to the porous nature of wood, which protects cells from direct irradiation.